Taking bulldozers for example, vehicles equipped with a power train designed to directly transmit the output of the engine to the transmission (gear shifter) through a main clutch (the vehicles of this type are hereinafter referred to as “direct-drive vehicles”) have superior power transmission efficiency, but require the operator driving skill of a certain level for effective operation because the load imposed on the vehicle needs to be controlled by operating the implement or the gear shifter. In contrast with this, vehicles equipped with a power train designed to transmit the output of the engine to the transmission through a torque converter (the vehicles of this type are hereinafter referred to as “torque-flow vehicles”) exert tenacious power during high load operation and enable easy gear shifting manipulation, but has inferior transmission efficiency and cannot carry out impactive rebating that is possible for the direct-drive vehicles.
There have been known torque-flow vehicles equipped with a lock-up system that make up for the functional shortcomings of the power trains of the vehicles of both types described above. In the torque-flow vehicles equipped with a lock-up system, a lock-up mode and a torque-converting mode are automatically selected according to the work load. In the lock-up mode, the lock-up clutch of the torque converter is actuated thereby directly transmitting the output of the engine to the transmission without passing through the torque converter. In the torque converting mode, the lock-up clutch is not actuated so that the output of the engine is transmitted to the transmission through the torque converter.
In the torque-flow vehicles equipped with a lock-up system, a speed change in the lock-up mode is carried out in such a way that after the lock-up mode has been switched to the torque converting mode, a gear shift is effected in the torque converting mode and then, the torque converting mode is again switched to the lock-up mode. As understood from the above, there exists a period during which the effective lock-up mode is not utilized. For further improving the efficiency of power transmission in the torque-flow vehicles equipped with a lock-up system, it is desirable to effect a gear shift in the lock-up mode without changing to the torque converting mode. However, when downshifting the transmission from a higher speed gear (e.g., second speed gear) to a lower speed gear (e.g., first speed gear), the rotational speed of the engine for the higher speed gear differs from the rotational speed of the engine for the lower speed gear at the shift point at which the driving performance (determined by the relationship between vehicle speed and tractive force) for the higher speed gear is equivalent to that for the lower speed gear, so that a change occurs in vehicle speed, leading to such a problem that the operator feels this change as a shock.
Many attempts have been made to solve the problem of such a shock caused by a speed change. For example, there has been proposed a technique (Japanese Patent Kokai Publication No. 5-229368) in which a shock caused by a speed change is alleviated by controlling the opening of the throttle so as to increase the rotational speed of the engine during the period from when a slip starts to occur in the friction clutch corresponding to the higher speed gear until when the friction clutch corresponding to the lower speed gear is perfectly engaged. According to another technique (Japanese Patent Kokai Publication No. 10-141487), a shock caused by a speed change is alleviated by controlling the pressure rising characteristics of the hydraulic friction clutch provided for each speed gear. There has been proposed still another technique (Japanese Patent Kokai Publication No. 2002-188713) according to which 24 speed ranges are offered by use of a Hi-Lo transmission, a primary transmission and a secondary transmission in combination and these transmissions are controlled so as to select a speed gear which provides a minimum speed difference at the time of gear shifting, whereby a shock caused by a speed change is mitigated.
The technique disclosed in Japanese Patent Kokai Publication No. 5-229368 has, however, proved to be unsuccessful in mitigating speed change shocks in bulldozers in which almost all operations are performed with the engine operating at full load (i.e., with the fuel throttle being in its full position), because this technique is intended for mitigation of speed change shocks in a driving condition where the engine is operated to output lower power than the output level of the full load operation (i.e., the operation with the fuel throttle fully opened). The technique disclosed in Japanese Patent Kokai Publication No. 10-141487 has also revealed a problem. Concretely, since this technique is designed to mitigate a speed change shock by moderating the degree of a gradual increase in hydraulic pressure working on a hydraulic friction clutch, the rise of driving torque is slow which causes a pause due to a torque shortage especially during high load operation. The technique associated with Japanese Patent Kokai Publication No. 2002-188713 presents the disadvantage of having a complex structure and control system.
The invention is directed to overcoming these problems and a primary object of the invention is therefore to provide a transmission system for a work vehicle, which is capable of reliably mitigating, with a comparatively simple arrangement, shocks caused by speed changes even in a work vehicle such as a bulldozer in which almost all operations are performed with the engine operating at full load.